Have you seen this new type AR lower?
They call it a Modular Weapon System, it has quick change mag wells.
You can change your AR to shoot any ammo from .22 to .50 Beowulf.
They were showing it at Knob Creek, and they have an add in Shotgun News Apr. 20 issue page 76. Here is there internet link.MGI Lower Receiver with Interchangeable Magazine Wells
Not a bad idea, but 10,000 rounds later will it still be a stable lower.
And for the price I could pick 5 of the cal.s that I wanted to shoot & buy lowers for those.

i have been gatherning parts for one for 5 or 6 years. i am not in a hurry i guess. it uses the ak mags. i think i have enough parts to finish it now. just to find the time to do it. a buddy of mine finished his. a picture of his.
his shoots great. i hope mine does also when i finish it.
(i bought 2 sets of uppers and lowers and sold him 1 set.)